Add cream to mushrooms; boil until thickened, about 1 minute. Mix in chopped tarragon. Season with salt and pepper.

Preheat broiler. Arrange salmon skin side down on broiler pan. Brush with lemon juice, then butter. Broil until just cooked through, without turning, about 6 minutes. Sprinkle with salt and pepper. Transfer to plates. Spoon Morels over.